Messages in this thread | | | Subject | Re: [PATCH] mm: Don't use radix tree writeback tags for pages in swap cache | From | Rik van Riel <> | Date | Thu, 25 Aug 2016 15:44:03 -0400 |
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On Thu, 2016-08-25 at 12:27 -0700, Huang, Ying wrote: > File pages use a set of radix tags (DIRTY, TOWRITE, WRITEBACK, etc.) > to > accelerate finding the pages with a specific tag in the radix tree > during inode writeback. But for anonymous pages in the swap cache, > there is no inode writeback. So there is no need to find the > pages with some writeback tags in the radix tree. It is not > necessary > to touch radix tree writeback tags for pages in the swap cache. > > With this patch, the swap out bandwidth improved 22.3% (from ~1.2GB/s > to > ~ 1.48GBps) in the vm-scalability swap-w-seq test case with 8 > processes. > The test is done on a Xeon E5 v3 system. The swap device used is a > RAM > simulated PMEM (persistent memory) device. The improvement comes > from > the reduced contention on the swap cache radix tree lock. To test > sequential swapping out, the test case uses 8 processes, which > sequentially allocate and write to the anonymous pages until RAM and > part of the swap device is used up. > > Details of comparison is as follow, > > base base+patch > ---------------- -------------------------- > %stddev %change %stddev > \ | \ > 1207402 ± 7% +22.3% 1476578 ± 6% vmstat.swap.so > 2506952 ± 2% +28.1% 3212076 ± 7% vm- > scalability.throughput > 10.86 ± 12% -23.4% 8.31 ± 16% perf-profile.cycles- > pp._raw_spin_lock_irq.__add_to_swap_cache.add_to_swap_cache.add_to_sw > ap.shrink_page_list > 10.82 ± 13% -33.1% 7.24 ± 14% perf-profile.cycles- > pp._raw_spin_lock_irqsave.__remove_mapping.shrink_page_list.shrink_in > active_list.shrink_zone_memcg > 10.36 ± 11% -100.0% 0.00 ± -1% perf-profile.cycles- > pp._raw_spin_lock_irqsave.__test_set_page_writeback.bdev_write_page._ > _swap_writepage.swap_writepage > 10.52 ± 12% -100.0% 0.00 ± -1% perf-profile.cycles- > pp._raw_spin_lock_irqsave.test_clear_page_writeback.end_page_writebac > k.page_endio.pmem_rw_page > > Cc: Hugh Dickins <hughd@google.com> > Cc: Shaohua Li <shli@kernel.org> > Cc: Minchan Kim <minchan@kernel.org> > Cc: Rik van Riel <riel@redhat.com> > Cc: Mel Gorman <mgorman@techsingularity.net> > Cc: Tejun Heo <tj@kernel.org> > Cc: Wu Fengguang <fengguang.wu@intel.com> > Cc: Dave Hansen <dave.hansen@intel.com> > Signed-off-by: "Huang, Ying" <ying.huang@intel.com> > --- > mm/page-writeback.c | 6 ++++-- > 1 file changed, 4 insertions(+), 2 deletions(-) > > diff --git a/mm/page-writeback.c b/mm/page-writeback.c > index 82e7252..599d2f9 100644 > --- a/mm/page-writeback.c > +++ b/mm/page-writeback.c > @@ -2728,7 +2728,8 @@ int test_clear_page_writeback(struct page > *page) > int ret; > > lock_page_memcg(page); > - if (mapping) { > + /* Pages in swap cache don't use writeback tags */ > + if (mapping && !PageSwapCache(page)) {
I wonder if that should be a mapping_uses_tags(mapping) macro or similar, and a per-mapping flag?
I suspect there will be another case coming up soon where we have a page cache radix tree, but no need for dirty/writeback/... tags.
That use case would be DAX filesystems, where we do use a struct page, but that struct page points at persistent storage, and the tags are not necessary.
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